This invention relates to magnetic domain stabilization of the read head of a merged type magneto-resistive head for a disk drive, including GMR (Giant Magneto-Resistive) read-write heads.
Disk drives are an important data storage technology. One of the crucial components of a disk drive are the read-write heads, which directly communicate with a disk surface containing the data storage medium. This invention corrects Electro-Static Discharge (ESD) damage to the pinned layer of the read head by the use of a write current applied to the write inductive coil and the use of a read current bias applied to the read head. The invention also corrects unstable read write heads, reducing base line popping.
FIG. 1A illustrates a typical prior art high capacity disk drive 10 including actuator arm 30 with voice coil 32, actuator axis 40, suspension of head arm 50 with slider/head unit 60 moving over disk surface 12.
FIG. 1B illustrates a typical prior art high capacity disk drive 10 with actuator 20 including actuator arm 30 with voice coil 32, actuator axis 40, head arms 50-54 and slider/head units 60-66 with the disks removed.
Since the 1980""s, high capacity disk drives 10 have used voice coil actuators including 20-66 to position their read-write heads over specific tracks. The heads are mounted on head sliders 60-66, which are included in a voice coil actuator and float a small distance off the disk drive surface 12 when in operation. Often there is one head per head slider for a given disk drive surface. There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator.
Voice coil actuators are further composed of a fixed magnet actuator 20 interacting with a time varying electromagnetic field induced by voice coil 32 to provide a lever action via actuator axis 40. The lever action acts to move head arms 50-56 positioning head slider units 60-66 over specific tracks with speed and accuracy. Actuator arms 30 are often considered to include voice coil 32, actuator axis 40, head arms 50-56 and head sliders 60-66. Note that actuator arms 30 may have as few as a single head arm 50. Note also that a single head arm 52 may connect with two head sliders 62 and 64.
Merged type heads possess different components for reading and writing, because the magneto-resistive effect only occurs during reading. A merged type head typically includes a thin film head and a spin valve sensor. The primary use of the thin film head is in the write process. The spin valve sensor is used for reading.
Merged Magneto-Resistive (MR) heads have several advantages over earlier approaches, using a single component, for both read and write. Earlier read-write heads were a study in tradeoffs. The single component, often a ferrite core, can increase read sensitivity with additional windings around the core. However, these added windings make the ferrite core write less efficiently.
Introduced in the 1990""s, merged heads brought significant increases in areal density. A merged type head reads the disk surface using a spin valve, containing a conductive thin film, whose resistance changes in the presence of a magnetic field. By separating the functions of writing and reading, each function can be optimized further than would be possible for the older read-write heads. For all the improvement that merged heads bring, there remain problems. However, before discussing these problems, consider first how and what controls these devices in contemporary disk drives.
FIG. 2A illustrates a simplified schematic of a disk drive controller 1000 controlling an analog read-write interface 220, write differential signal pair (w+ and wxe2x88x92), and the read differential signal pair (r+ and rxe2x88x92) communicating resistivity found in the spin valve within MR read-write head 200 of the prior art.
Note that usually the resistance of the read head is determined by measuring the voltage drop (V_rd) across the read differential signal pair (r+ and rxe2x88x92) based upon the read bias current setting Ir_set, using Ohm""s Law.
As illustrated in FIG. 2A, embedded disk controller 1000 includes computer 1100 accessibly coupled 1122 with memory 1120. Memory 1120 includes program system 1128. Embedded disk controller 1000 asserts Ir_set and Iw_set, both of which are presented to analog read/write interface 220. Iw_set is used by analog read/write interface 220 is control the write current presented to the write differential signal pair w+ and wxe2x88x92.
FIG. 2B illustrates a suspended head slider 60 containing the MR read-write head 200 of the prior art.
FIG. 2C illustrates a perspective view of merged read-write head 200 from FIG. 2B including write inductive head 202 and magnetoresistive read head (or spin valve) 204 of the prior art.
FIG. 2D illustrates a simplified cross section view of spin valve 204 with a region 206 composed of multiple layers forming the active region of spin valve 204 of FIG. 2C of the prior art.
FIG. 2E illustrates a more detailed cross section view of region 206 of FIG. 2D. a typical GMR spin valve of the prior art.
Region 206 contains Anti-FerroMagnetic (AFM) exchange film 208 deposited on pinned Ferro-Magnetic (FM) layer 210, over a copper (Cu) spacer layer 212 in turn deposited over free layer 214 on top of under layer 216 as typically found in a GMR spin valve of the prior art.
A GMR sensor is usually fabricated as follows: AFM layer 208 primarily composed of PtMn (Platinum Manganese). Pinned FM layer 210 is primarily composed of Co (Cobalt) NiFe (permalloy). The free layer 214 is primarily composed of NiFe permalloy. Under layer 216 is often composed primarily of Tantalum (Ta).
There is a distribution blocking temperature between layers 208 and 210. When the temperature of spin valve 204 exceeds the distribution blocking temperature, the exchange coupling between AFM layer 208 and FM pinned layer 210 vanishes.
During the manufacture and handling of spin valve 204, the magnetization of pinned layer (FM layer 210) may be reversed or rotated by 180 degrees due to an ESD event. The magnetization of the free layer may also be altered by an ESD event.
Note that the entire spin valve 204 is vertically located between shields S1 and S2 of FIG. 2C as will be illustrated in FIGS. 3A and 3B.
FIG. 2F illustrates normal magnetization of a spin valve read head as well as magnetization damage from ESD events as known in the prior art.
The AFM layer 208 will typically have a magnetization direction 300. Pinned layer 210 will normally have magnetization direction 310, but after one or more ESD events, may have a magnetization direction such as indicated by 312 or 314. The Cu spacer layer 212 is not specifically relevant in this discussion and is not illustrated here. Free layer 214 normally has a magnetization direction 320 and after damage from one or more ESD events, may have an altered magnetization direction as indicated by 322.
Normally, AFM layer 208 and pinned layer 210 have essentially parallel magnetization directions and free layer 214 is magnetized essentially perpendicular to layers 208 and 210. Operation of the spin valve read head 204 depends upon these directional relationships.
FIGS. 3A and 3B illustrate the magnetic flux direction related to the charging of the write differential signal pair connecting to P1 and P2, the poles of the write head, of the prior art.
FIG. 3A illustrates the magnetic flux D1 flowing from P1 to P2, when there is a positive write current asserted on the write differential signal pair under normal conditions in the prior art.
FIG. 3B illustrates the magnetic flux D2 flowing from P2 to P1, when there is a negative write current asserted on the write differential signal pair under normal conditions in the prior art.
Electro-Static Discharge (ESD) can diminish or damage the pinning part of the spin valve head 204 creating a weakened or reversed magnetic condition as discussed in FIG. 2F. Such conditions damage or destroy the ability of the spin valve 204, thus the MR read-write head 200 to function.
FIG. 4A depicts the ideal voltage amplitude measured across the read differential signal pair sensing a written pulse on a disk drive surface in the prior art.
As used in the prior art, the amplitude is defined as v++vxe2x88x92. Asymmetry is defined as v+xe2x88x92vxe2x88x92. The ideal situation would have a ratio of asymmetry to amplitude of 0%, but acceptable ranges are often 5% to 10%, with 7% being typical for a spin valve. ESD tends to decrease the amplitude and increase the asymmetry.
FIG. 4B illustrates base line popping, a condition often adversely affecting the quality of a spin valve and resulting from certain unstable read-write heads as known in the prior art.
Base line popping can lead to false detection of peaks (1) and troughs (0) as illustrated in FIG. 4B.
The prior art teaches repairing ESD damaged and unstable read heads by raising the read head temperature above the blocking temperature and generating a magnetic field across the read head. The prior art teaches applying a high read bias current to heat the read head, often using more than 10 mA, which may melt the read head. Sometimes an external magnetic field is used, requiring an external magnet, its power supply, and mechanical infrastructure positioning the external magnet with respect to the mechanical housing of the read-write head.
The prior art approach to repairing ESD damaged and unstable read heads has both reliability and cost problems associated with it. The external magnet and its requirements add to the cost of repair and thus, manufacture.
To summarize, what is needed are repair circuits and methods reducing the cost and improving the reliability of repairing and thus manufacturing MR read-write heads, and products containing these read-write heads (head sliders, actuator arms, voice coil actuators and disk drives).
The invention includes a method and apparatus repairing read heads of merged magnetoresistive read-write heads without the use of external magnets nor the heating the read head exclusively using the read bias current. The invention addresses at least the problems found in the prior art approaches.
The invention includes a write current source applying a write current level onto the write differential signal pair causing the write head to induce a temperature rise in the read head. A magnetic field within the read head is created by read current source applying a read current level onto the read differential signal pair. The read current and write current are maintained for at least a time period to effect repair.
By not requiring an external magnet, the invention costs less than any prior art approach, as well as protecting the read head from melting.
Because there is no external magnet required and current levels are within normal tolerances, the invention may be used to repair ESD damaged read heads in an assembled disk drive.